Prelude written in fortnight going west (5)
I believe the answer is:
intro
'prelude' is the definition.
(I've seen this in another clue)
'written in fortnight going west' is the wordplay.
'written in' indicates the answer is hidden within the clue.
'going west' is a reversal indicator (letters heading to the west or left).
'ortni' can be found hidden inside 'fortnight'.
'ortni' in reverse letter order is 'INTRO'.
(Other definitions for intro that I've seen before include "Prelude" , "Preamble" , "Jazz piece" , "Instrumental part at start of song" , "Opening instrumental passage of song" .)